The present invention relates to a method for synchronizing display elements which are arranged in and distributed over a plurality of subscribers of a control system for the automated control of a technical installation.
By way of example, a pertinent control system is a programmable controller, as marketed under the trademark PSS® by the assignee and described in “PSSuniversal, Programmable Control Systems PSS®, System Description, No. 21256-EN-04”. This control system is used for the automated control of a technical installation, particularly but not exclusively for controlling a safety-critical process that requires failsafe operation in order to avoid a danger to people. In particular, it involves the monitoring and control of installations operating in an automated fashion that have a risk of injury to people arising from their operation. Usually, such an installation is safeguarded by means of protective fences, protective doors, light barriers, light grids, emergency off pushbuttons and other safety sensors. As soon as a protective door is opened, a light barrier is interrupted or an emergency off pushbutton is operated, the installation needs to be stopped or put into a safe state in another manner. This is usually achieved by virtue of a controller in failsafe form evaluating state signals from the protective doors, light grids, emergency off pushbuttons etc., in defined recurring time intervals and taking this as a basis for operating safety-relevant actuators, such as contactors, that are arranged in the power supply path for the electrical drives of the installation. Frequently, the sensors and actuators are physically remote from the controller. In the case of a relatively large installation having a large number of sensors and actuators, it is possible for the controller to be connected to the sensors and actuators via what is known as a field bus system. The field bus system allows the interchange of digital data that represent the states of the sensors and actuators and also control commands from the controller. An example of such a field bus system is SafetyBUS p or SafetyNet p
PSSuniversal includes a modular controller for standard and safety tasks. The controller comprises a header module and a plurality of input/output modules that are arranged to the right of the header module in strung-together fashion. The input/output modules communicate with the header module via a module bus (also called backplane bus). The system provides many options for diagnosis. By way of example, diagnosis can be made by means of LEDs on the input/output modules and the header module.
The LEDs can twinkle or flash, for example in a particular rhythm, to indicate a certain status. It is thus possible to indicate a particular operating state of the system. One disadvantage is that said modules do not twinkle or flash in sync with one another. This can be irritating to the user, particularly because the modules are arranged close to one another or directly next to one another. In addition, out-of-sync twinkling or flashing can make simple “first-level” diagnosis difficult for the user.
There are methods for synchronizing subscribers in a network, particularly for synchronizing the time. By way of example, the brochure “SafetyNET p, Version 1.1, System Description” from Safety Network International e.V. (www.safety-network.de) discloses methods for synchronization between subscribers in the network. The methods initiate actions throughout the network at the desired instant, i.e. events synchronized on a network wide basis. In order to establish such methods on a network wide basis, each network subscriber requires an accurate clock that is in sync with the other subscribers. In this case, the accuracy of the clocks and of the synchronization determines the accuracy of the whole system. A measure that is used for the accuracy is jitter, the inaccuracy of the clocks in relation to one another. In the case of SafetyNET p, a high-precision master clock is used that uses synchronization mechanisms to continuously adjust the clocks of the other subscribers. The master clock itself can in turn be synchronized to world time atomic clocks. On the basis of precise, synchronous clocks, an action request can now be sent to the subscribers in advance, and these then perform the action at the required instant. Hence, transmission and execution are decoupled in time. This allows synchronization of the reading-in or output of data throughout the network. In SafetyNET p, all devices can synchronize the device clocks if this is necessary for the respective application. Synchronization of the SafetyNET p devices in real time frame network (RTFN) involves the use of the precision time protocol (PTP) standardized in IEEE 1588. In addition, the precise clock synchronization protocol (PCS) is used, which is optimized for use in real time frame line (RTFL). By way of example, WO 2006/069691 A1 describes such an RTFL method.
Such methods for synchronizing the time or time control methods are very involved and may produce a high bus load, however.